In many environments it is necessary to determine the stability of a signal that may or may not contain information and base certain actions on the results of the determination. For example, contemporary telephone systems frequently include tone receivers for converting DTMF (dual-tone multi-frequency) signals into rotary dial pulse form for use by central offices only equipped to respond to rotary dial type pulses. One of the problems with DTMF signals is that the tones used to form such signals fall in the audio frequency band. This is required because of the audio frequency bandwidth limits of contemporary telephone systems. Regardless of the reason why DTMF signals lie in the audio frequency band, tone receivers adapted to receive DTMF signals and convert them into rotary dial type pulses can erroneously respond to non-DTMF signals that lie in the audio frequency band, the most common of which are speech signals. As a result, it is desirable to inhibit the operation of tone receivers in the presence of non-DTMF audio signals and, in particular, speech signals. In order to accomplish this result it is necessary to be able to detect and respond to non-DTMF signals, such as speech signals, but not respond to DTMF signals. That is, a detector must be provided that can determine whether or not an incoming signal is a speech signal or a tone signal, and react accordingly to control the operation of the tone receiver so as to minimize errors resulting from non-DTMF signals, such as human voice signals. In this way, incorrect connections and, potentially, angry subscribers, can be avoided.
In the past, attempts to produce non-DTMF signal detectors, e.g., speech dectectors, having suitable accuracy have involved the application of concepts such as: measuring the ratio of the peak amplitude of the incoming signal to the average amplitude; and, measuring the distribution of energy in the incoming signals to determine if it is in the audio frequency band. These proposals have not been entirely satisfactory. First, the involved concepts have been difficult to implement in practical embodiments. Further, the implementations have tended to degrade the performance of the tone receiver, particularly in the presence of white noise.
Therefore, its is an object of this invention to provide a new and improved electronic detector.
It is a further object of this invention to provide an electronic detector suitable for discriminating between fluctuating signals, such as signals produced by the human voice and stable signals, such as DTMF signals.
It is another object of this invention to provide an electronic detector that is particularly suitable for use with a tone receiver to produce a speech immune tone detection system.
As will be better understood from the following description, the present invention provides a phase jitter detector for use in combination with a tone receiver to prevent the receiver from responding to fluctuating audio frequency signals, such as signals produced by the human voice. In addition to this use, the phase jitter detector includes the general ability to determine the level of noise in an incoming signal. That is, the phase jitter detector has the inherent ability to determine the signal-to-noise ratio (SNR) of a signal when its output is suitably interpreted. Since such information is useful in telephone and other electronic environments, it is also a broad object of this invention to provide a signal-to-noise ratio (SNR) measuring system.
In the past, SNR measuring systems have usually required that the output of a system in the absence of an input signal first be measured. This measurement represents the noise of the system. Then, a measurement of the output of the system in presence of a signal is made. The two measurements are then ratio combined to determine the SNR of the system. Obviously, it would be desirable to provide a system that provides SNR information in a single measurement step.
Therefore, it is a still further object of this invention to provide a new and improved SNR measuring system that determines signal-to-noise ratio in a single step.